The acquisition of data and subsequent generation of computer models for real-world objects is of interest in many industries and for many applications including architecture, physical plant design, entertainment applications (e.g., in movies and games), surveying, manufacturing quality control, medical imaging and construction, as well as cartography and geography applications. In order to obtain accurate 3D models of an object, as well as the area in which that object exists in the real world, it is necessary to take accurate measurements, or samplings of surfaces that make up the object, and elements of the surrounding area. Historically, this sampling was carried out using techniques that provided samples at the rate of tens or hundreds per hour at most.
Recent advances in scanning technology, such as technologies utilizing LIDAR scanning, have resulted in the ability to collect billions of point samples on physical surfaces, over large areas, in a matter of hours. In a LIDAR scanning process, the scanning device scans a laser beam across a scene that encompasses the structure of interest and the beam reflected from the scene is captured by the scanning device. The scanning device thus measures a large number of points that lie on surfaces visible in the scene. Each scan point has a measured location in 3D space, within some measurement error, that typically is recorded relative to a point (x,y,z) in the local coordinate system of the scanner. The resulting collection of points is typically referred to as one or more point clouds, where each point cloud can include points that lie on many different surfaces in the scanned view.
Conventional LIDAR scanning systems do not natively create points but instead create sets of ranges with associated mirror angles which are converted to x, y and z coordinates. The function which maps these native measurements into x, y and z coordinates depends on how the scanner was assembled and, for high accuracy systems, is different for each scanner and is a function of temperature and other environmental conditions. The differences between scanner systems are typically represented by a collection of numbers called calibration parameters. The purpose of a calibration system is to estimate the calibration parameters.
Calibration systems for scanners today typically involve measuring a collection of known targets and from these observations estimating the calibration parameters. The scanner system will measure the locations of one or more targets. These same targets locations are measured by a trusted reference system, for example, a total station calibrated using some other method. The calibration parameters at this measurement condition are then estimated. The aforementioned process might be carried out at one or more temperatures.
Such an approach suffers from at least three problems. First, a secondary measuring system is needed to locate the laser scanner targets. Second, if the targets move between the time the targets are measured with the secondary measuring system and when the targets are measured by the scanner being calibrated, perhaps because the targets are, for example, affixed to portions of a building which deform with sunlight or other factors over time, these motions will introduce systematic errors into the calibration parameters. Third, the ability to estimate the calibration parameters is no better than the ability to locate the targets with the scanner and the secondary measuring system. The further away a target is from the scanner the better the estimate of the angle portion of the calibration becomes. This has led the state of the art to large calibration systems with widely spaced targets. This requires a large stable space. The size of the space is costly and tends to aggravate the second problem; that is, targets remaining stable or fixed in position over time.
A total station is a manually operated optical instrument used in surveying. A total stations is a combination of an electronic theodolite (transit), an electronic distance meter (EDM) and software running on an external computer known as a data collector. With a total station one may determine angles and distances from the instrument to points to be surveyed. With the aid of trigonometry and triangulation, the angles and distances may be used to calculate the coordinates of actual positions (x, y, and z or northing, easting and elevation) of surveyed points, or the position of the instrument from known points, in absolute terms. Most modern total station instruments measure angles by means of electro-optical scanning of extremely precise digital bar-codes etched on rotating glass cylinders or discs within the instrument. The best quality total stations are capable of measuring angles down to 0.5 arcseconds. Inexpensive “construction grade” total stations can generally measure angles to 5 or 10 arcseconds.
Total stations solve the high accuracy angular calibration problem by using collimating telescopes. The total station to be calibrated is placed on a stable fixture. Telescopes, which have a target behind a set of lenses, are used as targets. By placing a set of lenses in front of their targets, the target appears to be located at a great distance, perhaps even hundreds of meters away when in fact the target is less than one meter away. This technique reduces the size of the angular portion of the total station calibration system to a few square meters. The reduced size also helps with the stability of the targets, since the targets are physically close together and can be mounted in the same stable base, typically made from concrete. Even the stability of the targets is not critical. The total station calibration system removes the need for a secondary measurement system or great stability by observing these targets in both faces.
In a two face measurement the total station is placed onto a stable mount. The operator then observes the targets in the telescopes through the telescope of the total station. The measurement is repeated in the second face; that is, by rotating the total station by 180 degrees, or one half of a revolution, on its base and repeating the measurements. The angular measurements are saved and from these observations both the locations of the telescopes and the relevant total station parameters can be determined without the use of a secondary measurement system.
The collimating telescope needs some adaptation to be used for laser scanners because laser scanners emit a laser beam whose location needs to be found. Additionally, the total station calibration method of using two face measurements and no secondary measurement system fails when applied to laser scanners. A typical laser scanner must move the laser beam very quickly. Generally this is done by using mirrors instead of by moving the laser. If the laser is moved entirely, the total station method of calibration is applicable because the uncertainty in the mounting of the laser beam behaves the same as the uncertainty in the elevation index and the total station collimation error. However, if mirrors move the laser beam, the uncertainty in the laser mounting are new parameters and the total station calibration method becomes ill conditioned for laser scanner calibration.
In summary, the state of the art in scanner calibration suffers from size, cost, and accuracy limitations because the calibration needs a secondary reference and accuracy is sensitive to the size and stability of the calibration system. The state of the art in total station calibration solves these problems but fails when adapted to laser scanner calibration. The total station calibration method fails to correctly identify all calibration parameters of the laser scanner because the way such laser scanners are physically arranged is different from total stations, i.e. because laser scanners use mirrors to deflect the laser beam.